1. Field of the Invention
The present invention relates to a screw-nut feed mechanism for effecting displacement of a movable table on a guiding base or bed in accordance with rotation of a feed screw shaft mounted on the guiding base.
2. Discussion of the Prior Art
In a conventional screw-nut feed mechanism, a ball nut assembly, which is threadedly coupled with a feed screw shaft, is fixedly attached to a movable table. In such case, horizontal deviation of the movable table is caused by a bad straightness (or whirling due to the bad straightness) of the feed screw shaft, an error in centering of the shaft or the lie, resulting in deterioration of straightness in feed of the movable table.
For solution of such a problem, a screw-nut feed mechanism has been proposed in Japanese Patent Publication No. 62-13861, wherein a ball nut assembly is carried on a movable table to be floatable in a plane perpendicular to the rotational axis of a feed screw shaft. In use of such a floating ball nut assembly, it is required to reliably restrict rotation of the ball nut assembly caused by rotation of the feed screw shaft, and it is also required to reliably apply a thrust force of the ball nut assembly to the movable table. These facts will have a great influence on feed precision of the movable table.
The floating mechanism disclosed in the above-mentioned Japanese Patent publication No. 62-13861 is provided with a cylindrical floating body having one end portion secured to the movable table, another end portion secured to one end of the ball nut assembly, and a mid portion arranged between the end portions and connected with them through a pair of axially spaced flexible portions, each of the flexible portions being formed to permit movement of the mid portion in a plane perpendicular to the axis of the feed screw shaft and to restrict rotation of the ball nut assembly. The floating mechanism is also provided with a pair of parallel tie-rods arranged in parallel with the axis of the feed screw shaft to transmit a thrust force of the ball nut assembly to the movable table.
The floating mechanism, however, has a problem that a pair of tie-rods are required for transmission of thrust force, thereby resulting in increase of the complexity of the floating mechanism and increase of the axial length thereof. The floating mechanism has another problem that the position of the movable table may change due to thermal expansion of the tie-rods.